Data Communications II, Autumn 2001

Problem set 5 (Tuesday 27.11.2001)

  1. Look for information about Gigabit Ethernet or even faster Ethernets in WWW or otherwhere. How is it possible to achieve gigabit transfer rates for Ethernets? Do these very fast Ethernets still use the same mechanisms as the slower ones? How else, except the speed, do the fast Ethernets differ from the slower ones?

  2. Token ring
    1. The token can get lost, because the station holding the token is disconnected from the network. How does a token ring handle the situation?
    2. Assume that in a token ring there is a fixed number, n, stations. Under what circumstances can be guaranteed that a station will get its turn to send after a limited maximum time? Under what circumstances no such maximum time exists? Does the token ring, in this respect, actually differ much from the the Ethernet LAN?
    3. In a token ring the sender removes the data frame. Would the token ring be more efficient if the receiver would remove the data frame and send a short acknowledgement to the sender?

  3. FDDI
    1. How does a FDDI LAN differ from a token ring?
    2. A large FDDI ring has 100 stations and a token rotation time of 40 msec. The token holding time is 10 msec. What is the maximum achievable efficiency of the ring?

  4. Mobile hosts
    1. A person with a portable computer travels to Berkeley and to his surprise finds out that there is a wireless LAN. So he doesn't have to plug his computer in, but can he use it? If he can, how is it done?
    2. n Berkeley there is also a GSM network. Supposing the local operator has a roaming contract with some Finnish operator (calls from one operator are transferred to another). Explain how a travelling person can read his email from Berkeley, or is it not possible?

  5. Find out about Bluetooth. What is Bluetooth? Where is it used? How is it related to other wireless standards? What OSI layer protocols does it standardise? How does it in prinzip function?

    1. Digitally encoded data is first packed into packets and then sent packet by packet forward. The time required to fill a packet is the packetization delay. Packetization delays greater than 20 ms can cause noticeable and unpleasant echo. A typical encoding and packetizing rate is 64 Kbps. How long is the packetizing delay for a maximum Ethernet frame of 1500 bytes? How long is the packetizing delay for a packet of 48 bytes.
    2. Why is atm using small fixed lenght packets?